Q.To secure A grade one must obtain an average of 90 marks or more in 5 subjects each of maximum 100 marks. If one scored 84, 87, 95, 91 in first four subjects, what is the minimum mark one scored in the fifth subject to get A grade in the course?

A linear inequality is solved exactly like a linear equation, with ONE crucial exception: multiplying or dividing both sides by a negative number reverses the inequality's direction; multiplying by a positive number never does.

Systems ('and'/'or'). Combining two inequalities with 'and' takes the intersection of their solution sets; combining with 'or' takes the union -- and when one solution ray is already a subset of the other, the union collapses to just the larger one (e.g. '$x<-1$ or $x<3$' is simply $x<3$).

Clearing fractions. Multiply both sides by the LCM of all denominators (a positive number, so no flip) before isolating the variable. …